Direct Current Link Circuit

ABSTRACT

An electronic circuit for converting power from a floating source of DC power to a dual direct current (DC) output is disclosed. The electronic circuit may include a positive input terminal and a negative input terminal connectible to the floating source of DC power. The dual DC output may connectible to the input of an inverter. A positive output terminal connected to the positive input terminal of the inverter and a negative output terminal and a ground terminal which may be connected to the input of the inverter. A series connection of a first power switch and a second power switch connected across the positive input terminal and the negative input terminal. A negative return path may include a first diode and a second diode connected between the negative input terminal and the negative output terminal. A resonant circuit may connect between the series connection and the negative return path.

TECHNICAL FIELD

Aspects of this disclosure relate to distributed power systems, particularly a photovoltaic power harvesting system and, more particularly to a direct current link circuit connected between a photovoltaic array and a 3-phase inverter circuit.

BACKGROUND

In a conventional photovoltaic power harvesting system configured to feed a single phase or a three phase alternating current (AC) power grid, dual (positive and negative) direct current (DC) power may be generated first from solar panels. The three phase inverter powered by the dual (positive and negative) DC power produces three phase AC power at the output of the three phase inverter. Conventionally, sufficiently high DC voltage may be provided to the input of the three phase inverter by connecting solar panels in series. However, in order to increase overall power conversion efficiency, the sum of positive and negative DC rails required by the inverter may be over 600 volts.

In North America, an input of voltage over 600 volts may create an issue with safety agency approval. An approach to avoid the safety issue may include inputting less than 600 volts to a boost circuit or transformer-isolated circuit to generate dual DC rails internally for the inverter input. The additional boost or transformer-isolated circuit increases cost and complexity especially since the additional power converter module generally requires dedicated control and protection features. Additionally, the boost or transformer-isolated circuit may also generate electromagnetic interference (EMI) and may cause reduction in overall efficiency of conversion of DC power to three phase AC power.

Thus there is need for and it would be advantageous to have a DC link circuit with a low voltage input, which does not cause significant reduction in overall efficiency of conversion of DC power to three phase AC power and which provides a sufficiently high DC input voltage to the AC inverter to generate an AC output of the inverter of required magnitude.

BRIEF SUMMARY

Embodiments include an electronic circuit for converting power from a floating source of DC power to a dual direct current (DC) output. The electronic circuit may include a positive input terminal and a negative input terminal connectible to the floating source of DC power. A positive output terminal and a negative output terminal and a ground terminal which may be connected to the dual DC output. The positive output terminal may be connected to the positive input terminal. The positive output terminal, the negative output terminal and the ground terminal may feed a three phase inverter. A charge storage device may be connected in parallel to the positive input terminal and the negative input terminal. The charge storage device may be charged from the positive input terminal and the negative input terminal.

A series connection of a first power switch and a second power switch connected across the positive input terminal and the negative input terminal. The series connection may provide a power output terminal between the first power switch and the second power switch and a negative return current path between the negative output terminal and the negative input terminal. The series connection may also include a first power terminal of the first power switch which connects to the positive output terminal and the positive input terminal. A second power terminal of the first power switch which connects to a third power terminal of the second power switch to provide the power output terminal. A fourth power terminal of the second power switch connects to the negative input terminal.

The negative return path may include a first diode and a second diode. The cathode of the first diode connects to the negative input terminal and the cathode of the second diode connects to the anode of the first diode to provide a diode terminal. The anode of the second diode connects to the negative output terminal and a resonant circuit may connect between the power output terminal and the diode terminal.

The resonant circuit may be adapted to alternately charge the resonant circuit and discharge the resonant circuit to the negative output terminal by an alternating switching signal applied to respective drive terminals of the first power switch and the second power switch. The alternating switching signal causes both the first power switch and the second power switch to turn on and turn off with substantially zero current.

Further embodiments include a second series connection of a third power switch and a fourth power switch. The second series connection may include a fifth power terminal of the third power switch connected to the positive output terminal and the positive input terminal. A sixth power terminal of the third power switch connected to a seventh power terminal of the fourth power switch to give a second power output terminal. An eighth power terminal of the fourth power switch connected to the negative input terminal. A third diode and a fourth diode connected in series between the negative output terminal and the negative input terminal. A cathode of the third diode connects to the negative input terminal. A cathode of the fourth diode connects to an anode of the third diode to give a second diode terminal. An anode of the fourth diode connects to the negative output terminal. A second resonant circuit connected between the second power output terminal and the second diode terminal. The second resonant circuit may be adapted to alternately charge the second resonant circuit and discharge the second resonant circuit to the negative output terminal by the alternating switching signal applied to respective drive terminals of the third power switch and the fourth power switch. The alternating switching signal causes both the third power switch and the fourth power switch to turn on and turn off with substantially zero current.

Embodiments include a method to convert power from a floating source of DC power to a dual direct current (DC) output with respect to electrical earth. The floating source of DC power may include a positive input terminal and a negative input terminal. The dual DC output may include a positive output terminal, a negative output terminal and a ground terminal. The positive input terminal connects to the positive output terminal. A cathode of a second diode and an anode of a first diode may be connected together. The anode of the second diode connects to the negative output terminal and the cathode of the first diode connects to the negative input terminal. The method charges a resonant circuit in a first switching cycle applied to a first power switch. The first switching cycle connects the resonant circuit across the positive input terminal and to the negative input terminal through the first diode. The resonant circuit discharges in a second switching cycle applied to a second power switch. The second switching cycle connects the resonant circuit in series between the negative input terminal and the negative output terminal through the second diode.

Embodiments include an electronic circuit for converting power from a floating source of DC power to a dual direct current (DC) output. The electronic circuit may include a positive input terminal and a negative input terminal connectible to the floating source of DC power. A positive output terminal and a negative output terminal and a ground terminal which may be connected to the dual DC output. The negative output terminal may be connected to the negative input terminal. The positive output terminal, the negative output terminal and the ground terminal may feed a three phase inverter. A charge storage device may be connected in parallel to the positive input terminal and the negative input terminal. The charge storage device may be charged from the positive input terminal and the negative input terminal.

A series connection of a first power switch and a second power switch connected across the positive input terminal and the negative input terminal. The series connection may provide a power output terminal between the first power switch and the second power switch and a positive return current path between the positive output terminal and the positive input terminal. The series connection may also include a first power terminal of the first power switch which connects to the positive output terminal and the positive input terminal. A second power terminal of the first power switch which connects to a third power terminal of the second power switch to provide the power output terminal. A fourth power terminal of the second power switch connects to the negative input terminal.

The positive return path may include a first diode and a second diode. The cathode of the first diode connects to the positive output terminal and the cathode of the second diode connects to the anode of the first diode to provide a diode terminal. The anode of the second diode connects to the positive input terminal and a resonant circuit may connect between the power output terminal and the diode terminal.

The resonant circuit may be adapted to alternately charge the resonant circuit and discharge the resonant circuit to the positive output terminal by an alternating switching signal applied to respective drive terminals of the first power switch and the second power switch. The alternating switching signal causes both the first power switch and the second power switch to turn on and turn off with substantially zero current.

Further embodiments include a second series connection of a third power switch and a fourth power switch. The second series connection may include a fifth power terminal of the third power switch connected to the positive output terminal and the positive input terminal. A sixth power terminal of the third power switch connected to a seventh power terminal of the fourth power switch to give a second power output terminal. An eighth power terminal of the fourth power switch connected to the negative input terminal. A third diode and a fourth diode connected in series between the positive output terminal and the positive input terminal. A cathode of the third diode connects to the positive output terminal. A cathode of the fourth diode connects to an anode of the third diode to give a second diode terminal. An anode of the fourth diode connects to the positive input terminal. A second resonant circuit connected between the second power output terminal and the second diode terminal. The second resonant circuit may be adapted to alternately charge the second resonant circuit and discharge the second resonant circuit to the positive output terminal by the alternating switching signal applied to respective drive terminals of the third power switch and the fourth power switch. The alternating switching signal causes both the third power switch and the fourth power switch to turn on and turn off with substantially zero current.

BRIEF DESCRIPTION OF THE DRAWINGS

Certain embodiments are illustrated by way of example, and not by way of limitation, in the accompanying figures, wherein like reference numerals refer to the like elements throughout:

FIG. 1 shows a photovoltaic power harvesting system according to conventional art.

FIG. 2 shows a power harvesting system in accordance with one or more embodiments described herein.

FIG. 3 shows a method for the power harvesting system shown in FIG. 2 according to one or more embodiments described herein.

FIG. 4 a shows a circuit according to one or more embodiments described herein.

FIG. 4 b shows a circuit which may be an interleaved topology version of the circuit shown in FIG. 4 a, according to according to one or more embodiments described herein.

FIG. 4 c shows a method, according to one or more embodiments described herein.

DETAILED DESCRIPTION

Reference will now be made in detail to features of the present invention, examples of which are illustrated in the accompanying figures. The features are described below to explain the present invention by referring to the figures.

Before explaining features of the invention in detail, it is to be understood that the invention is not limited in its application to the details of design and the arrangement of the components set forth in the following description or illustrated in the figures. The invention is capable of other features or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting. For example, the indefinite articles “a” and “an” used herein, such as in “a switch” and “a DC output” have the meaning of “one or more,” e.g., “one or more switches” and “one or more DC outputs.”

It should be noted, that although the discussion herein relates primarily to photovoltaic systems, the present invention may, by non-limiting example, alternatively be configured using other distributed power systems including (but not limited to) wind turbines, hydro turbines, fuel cells, storage systems such as battery, super-conducting flywheel, and capacitors, and mechanical devices including conventional and variable speed diesel engines, Stirling engines, gas turbines, and micro-turbines.

The term “switch” as used herein refers to any of: silicon controlled rectifier (SCR), insulated gate bipolar junction transistor (IGBT), bipolar junction transistor (BJT), field effect transistor (FET), junction field effect transistor (JFET), mechanically operated single pole double pole switch (SPDT), SPDT electrical relay, SPDT reed relay, SPDT solid state relay, insulated gate field effect transistor (IGFET), diode for alternating current (DIAC), and triode for alternating current (TRIAC).

The term “switch” as used herein refers to a three terminal device. Two out the three terminals referred to herein as “power terminals” and are equivalent to the collector and emitter of a BJT or the source and drain of a FET for example. The remaining “drive terminal” of the three terminal device being equivalent of the base of a BJT or the gate of a FET for example.

The term “positive current” as used herein refers to a direction of flow of a current from a higher potential point in a circuit to a lower potential difference point in the circuit. The term “negative current” as used herein refers to a flow of return current from a negative DC output to a negative input terminal.

The term “zero current switching” (or “ZCS”) as used herein is when the current through a switch is reduced to substantially zero amperes prior to when the switch is being turned either on or off.

The term “power converter” as used herein applies to DC-to-DC converters, AC-to-DC converters, DC-to-AC inverters, buck converters, boost converters, buck-boost converters, full-bridge converters and half-bridge converters or any other type of electrical power conversion/inversion known in the art.

The terms “power grid” and “mains grid” are used herein interchangeably and refer to a source of alternating current (AC) power provided by a power supply company and/or a sink of AC power provided from a distributed power system.

The term “period of a resonant circuit” refers to a time period of a substantially periodic waveform produced by the resonant circuit. The time period is equal to the inverse of the resonant frequency of the resonant circuit.

The term “low input voltage” is used herein refers to a floating (i.e., not referenced to a ground potential) DC voltage input across two terminals of less than 600 Volts or other voltage as specified by a safety regulation.

The term “dual DC” input or output refers to positive and negative terminals that may referenced to a third terminal, such as ground potential, electrical ground or a neutral of an alternating current (AC) supply which may be connected to electrical ground at some point.

The term “two level inverter” as used herein, may refer to its output. The AC phase output of the two level inverter has two voltage levels with respect to a negative terminal. The negative terminal is common to the AC phase output and the direct current (DC) input to the two level inverter. The alternating current (AC) phase output of the two level inverter may be a single phase output a two phase output or a three phase output. Therefore, the single phase output has two voltage levels with respect to the negative terminal. The two phase output has two voltage levels with respect to the negative terminal for each of the two phases. The three phase output has two voltage levels with respect to the negative terminal for each of the three phases.

Similarly, the term “three level inverter” as used herein may refer to an alternating current (AC) phase output of the three level inverter. The AC phase output has three voltage levels with respect to a negative terminal. The negative terminal is common to the AC phase output and the direct current (DC) input to the three level inverter. The alternating current (AC) phase output of the three level inverter may be a single phase output a two phase output or a three phase output. Therefore, the single phase output has three voltage levels with respect to the negative terminal. The two phase output has three voltage levels with respect to the negative terminal for each of the two phases. The three phase output has three voltage levels with respect to the negative terminal for each of the three phases.

The three level inverter compared with the two level inverter may have a cleaner AC output waveform, may use smaller size magnetic components and may have lower losses in power switches, since more efficient lower voltage devices may be used. Three level inverter circuits may have dual (positive and negative) direct current (DC) inputs.

Reference is made to FIG. 1, which shows a photovoltaic power harvesting system 10 according to conventional art. A photovoltaic string 109 includes a series connection of photovoltaic panels 101. Photovoltaic strings 109 may be connected in parallel together in an interconnected array 111, which provides a parallel direct current (DC) power output at DC power lines X and Y. The parallel DC power output supplies the power input of a direct-current-to-alternating-current (DC-to-AC) three phase inverter 103 on DC power lines X and Y. The three phase AC power output of inverter 103 (phases W, U and V) connects across an AC load 105. AC load 105 by way of example may be a three phase AC motor or a three phase electrical power grid.

Reference is now made to FIG. 2, which illustrates a power harvesting system 20 according to a feature of the present invention. System 20 includes interconnected photovoltaic array 111, which may provide a floating direct current voltage (DC) on positive input terminal A and negative input terminal B. The floating DC voltage may also be provided from other distributed power systems such as a DC voltage generator for example. Connected across positive and negative input terminals A and B is charge storage device C1, which may be a capacitor. Connected to positive input terminal A is the collector of an insulated gate bipolar transistor (IGBT) IGBT1. The emitter of IGBT1 connects to node C. IGBT1 may include an integrated diode with an anode connected to the emitter and a cathode connected to the collector. Connected to negative input terminal B is the emitter of an insulated gate bipolar transistor (IGBT) IGBT2. The collector of IGBT2 connects to node C. IGBT2 may include an integrated diode with an anode connected to the emitter and a cathode connected to the collector. Drive circuits G1 and G2 are connected to the bases of IGBT1 and IGBT2 respectively and may be referenced to ground. An inductor L1 connects between nodes C and D, where node D may connect to the ground and the ground input of inverter 103 a. A diode CR1 has an anode connected to positive input terminal A and a cathode connected to node V+. Diode CR1 provides a positive current path between nodes V+ and positive input terminal A. A capacitor C2 connects between node D and node V+. Node V+ provides a DC positive voltage to the input of inverter 103 a. A diode CR2 has a cathode connected to negative input terminal B and an anode connected to node V−. Diode CR2 provides a negative return current path between nodes V− and node B. Capacitor C3 connects between node D and node V−. Node V− provides a DC negative voltage to the input of inverter 103 a. Capacitors C2 and C3 may have substantially equal capacitance value. Inverter 103 a may have a 3 level inverter topology with dual DC input from nodes V+, V− and node D which may be converted to a single phase or a 3 phase AC voltage output, which supplies a load 105, which may be single phase or 3 phase load.

Reference is now made to FIG. 3, which shows a method 301 applied to power harvesting system 20 shown in FIG. 2, according to a feature of the present invention. In step 303, capacitor C1 may be charged by the floating DC voltage of array 111 by virtue of capacitor C1 being directly connected across array 111 at positive and negative input terminals A and B.

IGBT1 and IGBT2 may be gated alternately such that when IGBT1 is turned on, IGBT2 is off and vice versa by respective drive circuits G1 and G2. IGBT1 and IGBT2 may be gated alternately with less than a 50% duty cycle so as to avoid cross-conduction between IGBT1 and IGBT2 (i.e. to avoid IGBT1 and IGBT2 being on at the same time). A floating voltage provided from array 111 substantially provides a positive voltage on node V+ and a negative voltage on node V− with respect to the ground. The voltages on node V+ and node V− may be substantially equal to the magnitude of the floating voltage. Step 303, which charges capacitor C1 may continue during alternate gating of switches IGBT1 and IGBT2.

When switch IGBT1 is turned on (and IGBT2 turned off), current flows from array 111 and a discharge current flows (step 305 a) from storage capacitor C1 through collector and emitter of IGBT1, through inductor L1, into capacitor C3 and the input load of inverter 103 a between ground (node D) and node V−. Inductor L1 and capacitor C3 form a series resonant circuit. The diode across IGBT1 is reverse biased with respect to the voltage at positive input terminal A. The input voltage to inverter 103 a with respect to ground (node D) and node V− may be derived across capacitor C3. The resonant frequency of inductor L1 and capacitor C3 is given by Eq. 1 and the corresponding resonant periodic time T given in Eq. 2.

f ₀=1/2π(L1×C3)^(1/2)   Eq. 1

T=1/f ₀   Eq. 2

When IGBT1 initially turns on, there may be both zero current through inductor L1 and through the collector and emitter of IGBT1. After IGBT1 initially turns on, the current through L1 and the current through the collector and emitter of IGBT1 may increase and then fall sinusoidally. When IGBT1 turns off (the on period of the switch corresponds to half of the resonant periodic time T) there may be close to zero current through inductor L1 and through the collector and emitter of IGBT1.

A negative current path between node V− and negative input terminal B may be completed through diode CR2 corresponding to half of the resonant periodic time T.

Step 303 continues as capacitor C1 is still being charged by the floating DC voltage of array 111 by virtue of capacitor C1 being directly connected across array 111 at positive and negative input terminals A and B. When IGBT2 is turned on (and IGBT1 is turned off), current flows from array 111 and a discharge current (step 305 b) from storage capacitor C1 through diode CR1 through the input load of inverter 103 a between ground (node D) and node V+, through C2, through inductor L1 and through the collector and emitter of IGBT2. Inductor L1 and capacitor C2 form a series resonant circuit. The diode across IGBT2 may be reverse biased with respect to the voltage at negative input terminal B. The input voltage to inverter 103 a with respect to ground (node D) and node V+ is derived across capacitor C2. Capacitor C2 may have the same value as capacitor C3; therefore, the resonant frequency of inductor L1 and capacitor C2 and corresponding resonant periodic time T may be substantially the same. When IGBT2 initially turns on, there may be both zero current through inductor L1 and through the collector and emitter of IGBT2 and may be substantially zero power loss at turn on of IGBT2. After IGBT2 initially turns on, the current through L1 and the current through the collector and emitter of IGBT2 may increase and then fall sinusoidally. When IGBT2 turns off (the on period of the switch corresponds to half of the resonant periodic time T) there may be close to zero current in inductor L1 and close to zero current through the collector and emitter of IGBT2. Therefore, there may be zero power loss at turn off of IGBT2. A positive current path between node V+ and positive input terminal A is completed through diode CR1 corresponding to half of the resonant periodic time T. Zero current switching (ZCS) may, therefore, be provided for both turn on and turn off of both switches IGBT1 and IGBT2.

Zero current switching (ZCS) may permit the use and implementation of slower switching speed transistors for IGBT1 and IGBT2, which may have a lower voltage drop between collector and emitter. Thus, both switching losses and conduction losses may be reduced. Similarly, slower integrated diodes of IGBT1 and IGBT2 with lower voltage drop may be used. Slower diodes CR1 and CR2 may also be used. Resonant current shape through the collector and emitter of IGBT1 and IGBT2 may also reduce the turn-on losses in the diodes CR1 and CR2, as well as generated electromagnetic interference (EMI).

Another approach to generate dual DC rails, according to conventional art, may be to use boost or transformer-isolated circuits. If a boost circuit is used, the boost circuit conduction and switching losses may be very high. The boost inductor may be large and lossy and a reverse recovery problem of the output diode of the boost circuit may also be significant. Using a silicon carbide diode for the output of the boost circuit may remove the reverse recovery problem but may also increase the conduction loss. The overall cost of the boost circuit may be high if a number of expensive carbide diodes are paralleled together to accommodate high power levels. Also, some circuit topologies to generate dual DC rails from a solar panel may make the solar panel voltage vary with respect to ground. If the solar panel voltage is changing at a fast rate, ground circulating currents may be created and current levels set by safety agencies may be exceeded. The circuit topology described in various features and aspects below may address the above mentioned design considerations of circuit topologies to generate dual DC rails from a solar panel.

Reference is now made to FIG. 4 a which shows a circuit 40 a according to an aspect of the present invention. Interconnected photovoltaic array 111 is connected across capacitor C1 at nodes A and B. Connected to node A is the collector of transistor IGBT1. The emitter of IGBT1 is connected to the collector of transistor IGBT2 at node C. Both transistors IGBT1 and IGBT2 have an integral diode with an anode connected to the emitter and a cathode connected to the collector of each transistor respectively. Drive circuits G1 and G2 are connected to the bases of IGBT1 and IGBT2 respectively. The emitter of IGBT2 is connected to node B and the cathode of diode CR1. The anode of diode CR1 connects to the cathode of diode CR2 at node F. One end of inductor L1 connects to node C and the other end of inductor L1 connects to one end of capacitor C4. The other end of capacitor C4 connects to mode F. The anode of diode CR2 connects to the negative direct current (DC) input V− of DC to alternating current(AC) inverter 103 a. The anode of diode CR2 also connects to one end of capacitor C3, the other end of C3 connects to ground or neutral center-point node D. Node D connects to the ground input to inverter 103 a. One end of capacitor C2 connects to node D, the other end of capacitor C2 connects to node A and the positive direct current (DC) input V+ of DC to inverter 103 a. Inverter 103 a may have a 3 level inverter topology with dual DC input from nodes V+, V− and node D which may be converted to a single phase or a 3 phase AC voltage output which supplies a load 105 which may be single phase or 3 phase.

Alternately in circuit 40 a, diodes CR1 and CR2 may be placed in a series connection between node A and node V+. The series connection has the anode of diode CR2 connected to node A and the collector of IGBT1. The cathode of CR2 connected to the anode of diode CR1. The cathode of diode CR1 connected to node V+ and one end of capacitor C2. Tank circuit T1 still has one end of L1 connected to node C and the other end of L1 connected to one end of capacitor C4. The other end of C4 connects to the cathode of diode CR2. The emitter of IGBT2 and node B are now connected to node V− and one end of capacitor C3.

Reference is now made to FIG. 4 c which shows a method 401, according to a feature of the present invention. IGBT1 and IGBT2 in circuit 40 a are gated alternately with a pulse width modulation (PWM) cycle by drive circuits G1 and G2. IGBT1 and IGBT2 in circuit 40 a are gated alternately with up to almost 50% duty cycle so as to avoid cross conduction between IGBT1 and IGBT2. During the first half of the PWM cycle applied by drive circuit G1, IGBT1 is turned on at zero current (loss-less turn-on). Current then flows between the collector and emitter of IGBT1 into the series connected resonant tank T1 formed by inductor L1 and capacitor C4, diode CR1, and is returned to the negative input terminal (node B) of panel 111. Full solar panel 111 voltage V_(in) (across nodes A and B) is applied to resonant tank T1. As the current through the resonant tank T1 rises, capacitor C4 charges (step 403). When the voltage of capacitor C4 reaches the input voltage V_(in) (across nodes A and B), the current in tank T1 is reduced to be substantially zero. By the time IGBT1 turns off, the current through IGBT1 and tank T1 is already substantially zero, and turn-off of IGBT1 is also substantially loss-less.

During the second half of the PWM cycle IGBT1 is off and IGBT2 turns on at zero current. The charged capacitor C4 buffered by L1 is connected in series with the input voltage V_(in) (across nodes A and B). The voltage at the cathode of diode CR2 goes negative so that diode CR2 begins to conduct. A current path is formed from the positive input terminal (node A), through load and output filter capacitance provided by C2, C3 and inverter 103 a, through CR2, through capacitor C4 and inductor L1, through IGBT2 and to the negative input terminal (node B). The current path flowing through resonant tank T1 discharges (step 405) capacitor C4. Just as with IGBT1, both turn-on and turn-off of IGBT2 occurs at zero current, due to sinusoidal current in tank T1.

Resonant action of tank T1 may therefore allow the use of slower lower cost silicon output diodes CR1 and CR2 and possibly without reverse recovery problems of diodes used in conventional topologies to produce dual DC rails from a single DC source. Similarly, IGBT1 and IGBT2 can be slower, have lower voltage drop and therefore may be less expensive. Output voltage across terminals V+ and V− is substantially equal to twice the input voltage V_(in). With circuit 40 a no voltage feedback is needed to regulate the two DC outputs V+and V−.

Reference is now made to FIG. 4 b which shows a circuit 40 b which is an interleaved topology version of circuit 40 a shown in FIG. 4 a, according to an aspect of the present invention. The interleaved topology version has additional transistors IGBT3 and IGBT4, inductor L2, capacitor C5, diodes CR3 and CR4. Both transistors IGBT3 and IGBT4 have an integral diode with an anode connected to the emitter and a cathode connected to the collector of each transistor respectively. Connected to node A is the collector of transistor IGBT3. The emitter of IGBT3 is connected to the collector of transistor IGBT4 at node E. Drive circuits G1 and G2 are also connected to the bases of IGBT4 and IGBT3 respectively. The emitter of IGBT4 is connected to node B and the cathode of diode CR3. The anode of diode CR3 connects to the cathode of diode CR4 at node G. One end of inductor L2 connects to node E and the other end of inductor L2 connects to one end of capacitor C5. The series connection of inductor L2 and capacitor C5 forms resonant tank T2. The other end of capacitor C5 connects to mode G. The anode of diode CR4 connects to the negative direct current (DC) input V− of DC to alternating current (AC) inverter 103 a.

Alternately in circuit 40 b, diodes CR1, CR2, CR3 and CR4 may be placed in a series connections between node A and node V+. The series connection between CR1 and CR2 has the anode of diode CR2 connected to node A and the collectors of IGBT1 and IGBT3. The cathode of CR2 connected to the anode of diode CR1. The cathode of diode CR1 connected to node V+ and one end of capacitor C2. Similarly, the series connection between CR3 and CR4 has the anode of diode CR4 connected to node A and the collectors of IGBT1 and IGBT3. The cathode of CR4 connected to the anode of diode CR3. The cathode of diode CR3 connected to node V+ and one end of capacitor C2. Tank circuit T1 still has one end of L1 connected to node C and the other end of L1 connected to one end of capacitor C4. The other end of C4 connects to the cathode of diode CR2. The emitters of IGBT2, IGBT4 and node B are now connected to node V− and one end of capacitor C3. Similarly, Tank circuit T2 still has one end of L2 connected to node E and the other end of L2 connected to one end of capacitor C5. The other end of C5 connects to the cathode of diode CR4.

At high power, in conventional circuit topologies, semiconductor switches and output diodes usually may be paralleled together. In practice it may not feasible to parallel silicon diodes directly. Likewise, not all types of IGBTs may be paralleled directly also. Instead, as with circuit 40 b, the same number of switches and diodes may be re-arranged into interleaved topology, as shown in FIG. 4 b. Ripple current ratings of C1, C2 and C3 capacitors can be greatly reduced (along with cost and size) due to partial cancellation of ripple currents in them. The diodes CR1, CR2, CR3, CR4 and IGBT1, IGBT2, IGBT3 and IGBT4 share a load (input to inverter 103 a) without having to be paralleled directly, so sharing of power to be delivered to the load may no longer be an issue.

Although selected features of the present invention have been shown and described, it is to be understood the present invention is not limited to the described features. Instead, it is to be appreciated that changes may be made to these features without departing from the principles and spirit of the invention, the scope of which is defined by the claims and the equivalents thereof. 

1-15. (canceled)
 16. An electronic circuit comprising: positive and negative input terminals adapted to be connected across a floating source of direct current (DC) power; positive and negative output terminals; first and second switches connected in series across the positive and negative input terminals, wherein a connection point between the first and the second switches forms a first node; first and second diodes connected in series, wherein a connection point between the first and the second diodes forms a second node; and a resonant circuit connected in series between the first and the second nodes, wherein, when the first switch is closed and the second switch is open, the resonant circuit is connected across the positive and then negative input terminals through the first diode, and wherein, when the first switch is open and the second switch is closed, the positive and negative output terminals and the resonant circuit are connected in series across the positive and negative input terminals through the second diode.
 17. The electronic circuit of claim 16, wherein a cathode of the first diode is connected to the negative input terminal, an anode of the second diode is connected to the negative output terminal, and an anode of the first diode and a cathode of the second diode are connected to the second node.
 18. The electronic circuit of claim 16, wherein an anode of the first diode is connected to the positive input terminal, a cathode of the second diode is connected to the positive output terminal, and a cathode of the first diode and an anode of the second diode are connected to the second node.
 19. The electronic circuit of claim 16, further comprising a charge storage device connected across the positive and negative input terminals.
 20. The electronic circuit of claim 16, wherein the resonant circuit comprises an inductor and a capacitor connected in series between the first and second nodes.
 21. The electronic circuit of claim 16, further comprising: a ground output terminal; a first capacitor connected between the positive and ground output terminals; a second capacitor connected between the negative and the ground output terminals; and an inverter having a positive inverter input terminal connected to the positive output terminal, a negative inverter input terminal connected to the negative output terminal, and a ground inverter terminal connected to the ground output terminal.
 22. The electronic circuit of claim 16, further comprising first and second drive circuits adapted to gate the first and the second switches alternatively with a pulse width modulation (PWM) cycle such that the first switch is closed while the second switch is open during a first half of the PWM cycle and the second switch is closed while the first switch is open during a second half of the PWM cycle.
 23. The electronic circuit of claim 22, wherein the first and the second drive circuits are adapted to gate the first and the second switches alternatively with less than a fifty percent duty cycle so as to avoid cross conduction between the first and second switches.
 24. The electronic circuit of claim 22, wherein the first and second drive circuits are configured to open and close the first and second switches with substantially zero current through the switches.
 25. The electronic circuit of claim 16, further comprising: third and fourth switches connected in series across the positive and negative input terminals, wherein a connection point between the third and the fourth switches forms a third node; third and fourth diodes connected in series, wherein a connection point between the third and the fourth diodes forms a fourth node; and a second resonant circuit connected in series between the third and the fourth nodes, wherein, when the third switch is closed and the fourth switch is open, the second resonant circuit is connected across the positive and then negative input terminals through the third diode, wherein, when the third switch is open and the fourth switch is closed, the positive and negative output terminals and the second resonant circuit are connected in series across the positive and negative input terminals through the fourth diode, wherein the first and the fourth switches are configured to be opened and closed together, and wherein the second and third switches are configured to be opened and closed together.
 26. A method for converting power from a floating source of direct current (DC) power to a dual DC output having positive and negative output terminals referenced to a ground output terminal, the method comprising: connecting the floating source of DC power across positive and negative input terminals of a circuit, the circuit including: first and second switches connected in series across the positive and negative input terminals, wherein a connection point between the first and the second switches forms a first node, first and second diodes connected in series, wherein a connection point between the first and the second diodes forms a second node, and a resonant circuit connected in series between the first and the second nodes, wherein, when the first switch is closed and the second switch is open, the resonant circuit is connected across the positive and negative input terminals through the first diode, and wherein, when the first switch is open and the second switch is closed, the positive and negative output terminals and the resonant circuit are connected in series across the positive and negative input terminals through the second diode; gating, alternatively, the first and second switches such that the first switch is closed and the second switch is open during a first phase of a pulse width modulation (PWM) cycle thereby charging the resonant circuit from the floating source of DC power, and such that the first switch is open and the second switch is closed during a second phase of the PWM cycle thereby discharging the resonant circuit to provide converted power to a load connected to the positive, negative, and ground output terminals.
 27. The method of claim 26, wherein the first and second diodes are connected in one of the following manners: a cathode of the first diode is connected to the negative input terminal, an anode of the second diode is connected to the negative output terminal, and an anode of the first diode and a cathode of the second diode are connected to the second node; and an anode of the first diode is connected to the positive input terminal, a cathode of the second diode is connected to the positive output terminal, and a cathode of the first diode and an anode of the second diode are connected to the second node.
 28. The method of claim 26, wherein the load includes an inverter, the method further comprising: inverting the converted power with the inverter.
 29. The method of claim 26, further comprising gating, alternatively, the first and second switches with less than a fifty percent pulse width modulation duty cycle so as to avoid cross conduction between the first and second switches.
 30. The method of claim 26, further comprising gating, alternatively, the first and second switches such that the first and second switches open and close with substantially zero current through the switches. 